# Car battery

#### EFLImpudence

How do you work out the voltage of a car battery with a timed drain?

For example, I have detected a 100mA drain in addition to that deemed acceptable with all the electronics.
Would that be enough to explain a 70Ah battery not starting after, say, 20 hours or 40 hours?

It seems to go down about half a volt per day.

Is it definitely worn out?

#### securespark

If you disconnected it from the wiring, how rapid is the discharge then?

#### EFLImpudence

Haven't tried that yet.

Will charge it up a bit tomorrow and then do that.

#### bernardgreen

It should be possible to take 100mA for 700 hours before the battery voltage is reduced to the rated end point voltage ( 70Ah = 70,000 milli-amp hours ).

The rated end point depends on the battery and the application.

From memory for lead acid batteries in vehicles the rated end point is around 0.85 of the nominal voltage when a normal current is being drawn from the battery

Hence a 12 volt battery is likely to have a rated end point of 10.2 volts. The hundreds of amps needed to start the vehicle engine is not considered a normal current load and the battery voltage of a fully charged battery may drop below the end point voltage when starting the vehicle

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#### EFLImpudence

So 150mA (including the electronics) for 20 hours should not have enough effect to stop it starting.

Definitely a duff battery, then?

#### Spark123

Charge it up and take it to a garage and get them to do a drop test, this is the easiest way to do check the cold cranking amps cca of the battery.

#### JohnW2

So 150mA (including the electronics) for 20 hours should not have enough effect to stop it starting. Definitely a duff battery, then?
I agree with the others it's duff. Assuming that you are talking about starting with it as fully charged as you can get it, a serviceable 70 Ah battery should hardly notice the 3Ah, over 20h, you're talking about.

Kind Regards, John

#### EFLImpudence

I've ordered a new one but as is the way with the internet - it could be this; it could be that.

I was just wondering how you calculate the drop in voltage and indeed if there should be one or should it stay at ~12V but just get weaker.
Plus, why, if you jump start it, is it alright for the rest of the day but not the next morning?

I have been fiddling with cars for fifty years. Maybe I haven't had time to think about it before.

#### EFLImpudence

should hardly notice the 3Ah, over 20h, you're talking about.
Typo 'h' ?

#### JohnW2

JohnW2 said:
should hardly notice the 3Ah, over 20h, you're talking about.
Typo 'h' ?
No, but perhaps I should have made my point more clearly. 150mA for 20h is 3Ah - which is a pretty tiny proportion of the battery's 70Ah capacity. I suspect what you're suggesting is that the "over 20h" is redundant. That's not quite true, and I was making the point that the (already small) load was spread over a very reasonable period of time. If the same "3Ah" had been drawn in 1 minute (hence a current of 180A), rather than 20 hours, then things would probably be a bit different - the behaviour of batteries is far from 'linear'!

Kind Regards, John

#### endecotp

I was just wondering how you calculate the drop in voltage

It's not linear, there's no easy calculation. You can probably find graphs of typical voltage vs. % discharge though.

#### EFLImpudence

John:

Yes, I realised since I wrote it.

However, with the three times the acceptable drain, after, say, a fortnight (50Ah instead of 16) it would be considerable so should be investigated.

#### JohnW2

I was just wondering how you calculate the drop in voltage and indeed if there should be one or should it stay at ~12V but just get weaker.
I'm not sure what you mean by 'getting weaker'. If you mean that it's current-supplying capacity is weaker, then applying an appreciable load would cause the on-load voltage to fall further.

You're asking a complicated question, about the voltage of a rechargeable battery as it discharges. This will vary consdierably according to the type (chemistry) of the battery - whilst the more modern types (NiCad NiMh etc.) tend to maintain their nominal voltage throughout most of their discharge life, and then 'suddenly plummet', the voltage of lead-acid batteries such as you're talking about tends to reduce progressively throughout discharge. It's therefore not something you can 'calculate' directly - you need to look at the relevant 'curves' for the type of battery concerned, and an added complication is that those curves will (as hinted in my recent post) vary a lot depending upon the 'rate of discharge'.
Plus, why, if you jump start it, is it alright for the rest of the day but not the next morning?
I'm not quite sure what you are describing. If you jump start a car which has an essentially flat battery, it will obviously then run (from it's alternator) regardless of the battery. The battery will also get charged to some extent whilst the engine is running, usually enough to start the engine again if it is stopped for a relatively short time. However, if it doesn't get charged up all that much, it might not be man enough to start the engine when it is 'stone cold' the next morning.

Kind Regards, John

#### JohnW2

John:Yes, I realised since I wrote it.
Fair enough.
However, with the three times the acceptable drain, after, say, a fortnight (50Ah instead of 16) it would be considerable so should be investigated.
I'm struggling a bit with that - could you perhaps find another way of wording it? !!

Kind Regards, John

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